The Church of San Pietro was originally annexed to the Augustinian convent, which settled in Leonessa in the 13th century. The façade, made of square stone masonry with a horizontal crowning, has a round portal in red stone, dated 1467, surmounted by a keystone tympanum. In the sculptural decoration of the architrave is inscribed in bas-relief the city coat of arms, characterised by a rampant lion embracing a P, symbolising the proudly claimed sovereignty of the people.
The bell tower is aligned with the façade and has three orders. The articulated arrangement of the elements makes it possible to identify the progression of building interventions over the centuries. The church has a Latin cross plan, originally with three naves. After the earthquake of 1703, the church was rebuilt with a single nave, at the sides of which four chapels open symmetrically.